Until recently, helium gas has been considered devoid of cardiovascular actions. However, over the past three years, there have been reports that breathing a helium-oxygen mixture induces a protection against the arrythmic effect of coronary ligation in dogs. At this stage, it is not known whether the sympathetic nervous system is involved in this action, or whether helium has a similar effect against arrthmias induced by other ways. We propose to examine whether breathing helium - oxygen has an antagonistic effect on arrythmias and fibrillations induced in rats by coronary artery ligation and by drugs. In addition, we intend to study the changes in cardiac dynamics induced in dogs by ligation of the left coronary circumflex artery in the presence and absence of helium. In the event that we find cardiovascular actions attributable to respiring helium, we will extend the research to the isolated perfused heart and isolated vascular smooth muscle preparations. In this way, we should also be able to examine whether any changes are induced in catecholamine release or metabolism; likely candidates in any anti-arrythmic mechanism. The above approach would establish: 1) whether helium does posses anti-arrythmic actions and if so, 2) the mechanism(s) of this action.